The average adult's body weight is generally 50 to 60 percent water—enough, if it were bottled, to fill 40 to 50 quarts. For example, in a 150pound man, water accounts for about 90 pounds and fat about 30 pounds, with protein, carbohydrates, vitamins, and minerals making up the balance. Men generally have more water than women, a lean person more than an obese person. Some parts of the body have more water than others. Human blood is about 92 percent water, muscle and brain tissue about 75 percent, and bone 22 percent.
The body uses water for virtually all its functions: digestion, absorption, circulation, excretion, transporting nutrients, building tissue, and maintaining temperature. Almost all body cells need and depend on water to perform their functions. Water carries nutrients to the cells and carries away waste materials to the kidneys.
Water is needed in each step of the process of converting food into energy and tissue. Water in the digestive secretions softens, dilutes, and liquefies the food to facilitate digestion. It also helps move food along the gastrointestinal tract. Differences in the fluid concentration on either side of the intestinal wall enhance the absorption process.
Water serves as an important part of body lubricants, helping to cushion the joints and internal organs; keeping tissues in the eyes, lungs, and air passages moist; and surrounding and protecting the fetus during pregnancy.
Many adults take in and excrete between 8 and 10 cups of fluid daily. Nearly all foods have some water. Milk, for example, is about 87 percent water, eggs about 75 percent, meat between 40 and 75 percent, vegetables from 70 to 95 percent, cereals from 8 to 20 percent, and bread around 35 percent.
The body gets rid of the water it doesn't need through the kidneys and skin and, to a lesser degree, from the lungs and gastrointestinal tract. Water is also excreted as urine by the kidneys along with waste materials carried from the cells. About 4 to 6 cups a day are excreted as urine. The amount of urine reflects, to some extent, the amount of an individual's fluid intake, although despite the amount consumed, the kidneys will always excrete a certain amount each day (about 2 cups) to eliminate waste products generated by the body's metabolic actions. In addition to the urine, air released from the lungs contains some water, and evaporation that occurs on the skin (when sweating or not sweating) contains water as well.
If normal and healthy, the body maintains water at a constant level. A number of mechanisms, including the sensation of thirst, operate to keep body water content within narrow limits. You feel thirsty when the blood starts to become too concentrated. Unfortunately, by the time you feel thirsty, you are already much in need of extra fluid. It is therefore very important not to ignore feelings of thirst, a concern that is particularly appropriate for the elderly, whose thirst mechanism is compromised. The well-known recommendation to drink 8 cups of fluid daily is too much for some, like many elderly, and too little for others, like athletes.
There are, of course, conditions in which the various body mechanisms for regulating water balance do not work, such as severe vomiting, diarrhea, excessive bleeding, high fever, burns, and excessive perspiration. In these situations, large amounts of fluids and minerals are lost. These conditions are medical problems to be managed by a physician.